As is well known, spectrometry is a technique wherein light is directed onto a substance to be analyzed, and the resulting light transmitted, reflected, and/or emitted by the sample is then analyzed to provide information about the substance. Vibrational circular dichroism (VCD), or VCD spectroscopy, is a spectrometric technique which uses circularly polarized light to provide information about a substance. As is well known, light is formed from an oscillating electric field—an electric “wave”—and when the field/wave oscillates in a particular way, it is said to be polarized. For example, light having a field oscillating in a plane is said to be plane polarized. Circularly polarized light is then formed when the field oscillates in two perpendicular planes, with the waves in each plane being out of phase, such that the peak of the waves appears to spiral along their direction of travel. When the spiral is oriented counterclockwise when traveling toward a viewer, the light is said to be right-circularly polarized; conversely, if spiraling clockwise, the light is said to be left-circularly polarized.
An interesting feature of many substances is that they respond differently to incident light having different polarization—they may absorb, reflect, and/or transmit different amounts of differently-polarized light. VCD techniques are generally directed to determining the difference in absorption that a substance exhibits between right and left circularly polarized light. VCD measurements are particularly useful in the field of stereochemistry, i.e., the study of the shapes of molecules and the spatial arrangement of atoms therein. More particularly, VCD measurements are useful in the study of substances which contain chiral molecules—molecules having structures which cannot be superimposed on their mirror images. (The concept of chirality is illustrated by a person's right hand, which can be said to be chiral: it is a mirror image of their left hand, but the hands cannot be superimposed no matter how one orients them relative to each other.) As an example, many substances, particularly biological substances, contain chiral molecules of opposite senses—that is, the molecules are mirror images of each other (in which case they are known as enantiomers or optical isomers). Each of the enantiomers may have different properties, in particular, different biological response—for example, sugars are chiral molecules, and the human body can digest and use “right-handed” sugars, but not their left-handed counterparts. Since VCD spectral bands of enantiomers have opposite sign, VCD spectroscopy can allow one to differentiate between enantiomers, a result which is extremely useful in pharmaceutical and chemical fields, among others. Similarly, one can determine how much of one enantiomer is present with respect to its twin, by looking at the spectrum of the mixture of enantiomers and comparing it to one of the “pure” enantiomers (since the difference will reflect how much the spectrum of one enantiomer attenuates the other). Further details on VCD spectrometers, VCD spectrometric techniques, and the uses thereof can be found, for example, in U.S. Pat. No. 6,480,277 to Nafie and in B. Wang, American Laboratory, 36C-36P (April, 1996).
However, VCD spectrometry has numerous drawbacks, some of the most significant being the very small magnitude of the VCD signal from a sample to be analyzed (on the order of 10-4−10-5 absorbance units), and the tendency for measurements to be prone to noise and artifacts (which arise in part owing to the 16w signal magnitude). An associated problem is the time needed to generate a high-quality VCD spectrum: owing to the low signal-to-noise ratio (SNR) in conventional VCD arrangements, VCD measurements must usually be generated in multiple scans (i.e., repeated measurements must be taken), with the measurements then being averaged or otherwise processed to achieve better SNR. These scans can take significant time to execute, with scans sometimes requiring several hours, which is inconvenient. Further, as a result of the need for numerous scans (and significant scan times), VCD measurements of “transient” samples—samples which change over time, e.g., reacting mixtures—are usually not feasible.